This invention relates to a method and an apparatus capable of satisfactorily conducting reduction of a thin slab, particularly a hot thin slab to a considerable extent in the widthwise direction so as to decrease the width thereof.
As a conventional method of adjusting the breadth or width of a hot slab, there is the method of conducting the reduction of a relatively thick slab (having a thickness of 200 mm and a width of 1500 mm for example) by vertical type rolls installed in front of a group of rough rolling mills of a hot rolling equipment in the widthwise direction by use of an equipment described in Japanese Patent Kokai (Laid-Open) No. 114501/81.
However, according to this method of adjusting the width, the value of width decrease by one pass was limited to 100 mm, whereby, in order to decrease the plate width of the slab from 1500 mm to 1300 mm (i.e. the value of width decrease is 200 mm), it was necessary to conduct two passes or more. Moreover, since the rolling was made by the vertical type rolls, extremely thick portions (hereinafter referred to as "dogbones") occurred at portions adjacent opposite ends of plate width. If horizontally rolled thereupon, then the dogbone portions flowed, spreading in the plate widthwise direction, which were called width spread phenomena. Thus, there was presented such a drawback that the efficiency of width decrease was lowered.
Furthermore, at the forward and rear ends of the slab, there occurred the longitudinal expansions of only opposite end portions in the plate widthwise direction, the so-called fishtails. For this, the dogbones are lessened in size at the front and rear ends of the slab at the same time. Therefore, the width spread phenomena by the subsequent horizontal rollings are small in value at the forward and rear ends of slab. Hence, there have been presented such disadvantages that the slab is lessend in its width and the fishtails are further expanded, so that the yield is lowered to a considerable extent. Further, recently, there has been a demand for continuous casting of slab of thin thickness (thickness of 30 to 50 mm for example). In this case, with the above-described method, there has been presented such a drawback that a buckling tends to occur in the widthwise direction of slab.
On the other hand, there is a method of changing the width of slab in a continuous casting equipment, and, in many continuous casting equipments, moulds are replaced to change the width of slab. However, this method has been considerably disadvantageous in that the casting operation should be stopped during the replacement of the moulds, whereby the casting efficiency is low and the production is lowered. Therefore, the replacement of the moulds made it difficult to efficiently manufacture slabs of various widths.
Recently, there has been developed a technique, according to which the width of slab is changed during casting, without the replacement of moulds. However, there still remain the problems in the material quality of the surface of cast slab and the leakage of molten steel. Further, if the change of width is made suddenly, then there occurs a problem of a breakout, etc. due to an incomplete formation of a solidified shell. Hence, the change of width has had to be made slowly. In consequence, the slab in the widthwise direction during the change of width is formed into a tapered shape, and, even at a casting speed of about 1 m/min, the length of a portion being of the tapered shape reaches about 10 m or more when the width of slab is changed from 1500 mm to 1300 mm.
Further, recently, the adoption of high casting speed has been studied. In this case, the tapered portion is further lengthened. In consequence, there are presented such disadvantages that it becomes necessary to adjust the tapered portion in its width during a later process, whereby the number of man-hours must be increased.
As described above, the conventional method of decreasing the width of a hot slab, particularly a thin slab, either in the rolling by the vertical type rolls or by the means of changing the width in the continuous casting equipment, has been disadvantageous in that the decrease of width cannot be carried out efficiently.
In addition, as a system for decreasing the occurrences of fishtails in the rollings in the widthwise direction of slab, such an apparatus is well known that a press device for decreasing the width of the end portions in the plate widthwise direction of slab is provided upstream of the vertical type rolls as shown in Japanese Patent Kokai (Laid-Open) No. 68504/81.